The present invention relates to a semiconductor device including a fuse circuit which can be disconnected by laser ablation and a method for fabricating the semiconductor device, and a laser system suitable to disconnect a fuse of the semiconductor device.
Semiconductor devices, such as memory devices of DRAMs, SRAMs, etc., logic devices, etc., are constituted by a very large number of elements, and a part of the circuit or of the memory cells are often inoperative due to various cause in their fabrication processes. In this case, when semiconductor devices partially defective circuits or memory cells are generally regarded as defective devices, the semiconductor devices have low fabrication yields, which might lead to fabrication cost increase. In view of this, recently such defective semiconductor devices have defective circuits or defective memory cells replaced by redundant circuits or redundant memory cells which have been prepared in advance, to create properly functioning devices. In some semiconductor devices, a plurality of circuits having functions different from each other are formed integrated and later those of certain functions are replaced, and in other semiconductor devices prescribed circuits are formed, and later characteristics of the semiconductor devices are adjusted. In such reconstruction of semiconductor devices, usually a fuse circuit having a plurality of fuses is formed on the semiconductor devices, and after operation tests, etc., the fuses are disconnected by laser beam irradiation.
A conventional semiconductor device including a fuse circuit and a method for fabricating the same will be explained with reference to FIGS. 11A-11C. FIG. 11A is a diagrammatic sectional view of the conventional semiconductor device, which shows a structure thereof. FIG. 11B is a plan view of the conventional semiconductor device, which shows the structure thereof. FIG. 11C is a diagrammatic sectional view of the conventional semiconductor device with a fuse disconnected, which shows the structure thereof.
A fuse 202 is formed on a substrate 200, connected to a prescribed circuit for replacing the circuit. An inter-layer insulation film 204 for covering the fuse 202 is formed thereon. An interconnection layer 206 is formed on the inter-layer insulation film 204, connected to the fuse 202 therethrough. A passivation film 211 is formed on the interconnection layer 206. A part of the passivation film 211 on the fuse 292 is removed. A plurality of the fuses 202 are formed on the substrate 200 at a prescribed pitch (FIGS. 11A and 11B).
To disconnect the fuse 202 in such fuse circuit, a laser beam 208 is irradiated to a region where the fuse is formed, whereby the fuse 202 is rapidly heated by its absorbed energy to a high temperature and undergoes laser explosion (FIG. 11C).
Here to further micronize the semiconductor device, it is necessary to further decrease a pitch between the fuses 202, but a pitch P of the fuses 202 is determined by a spot size 210 of the laser beam 208 and alignment accuracy of the laser beam 208.
A spot size of the laser beam 208 has a lower limit which is determined by a wavelength of the laser beam 208, and the spot size 208 can be decreased as the laser beam has a shorter wavelength. However, when a wavelength of the laser beam is too short, there is a risk that the laser beam may pass through a region where the fuse 202 is not formed, arrives at the base semiconductor substrate and is absorbed therein, and cause thermal laser explosion. In a case that the semiconductor substrate is silicon, the laser beam has an about 1 xcexcm wavelength, at which silicon substrates absorb small amounts of laser beams. That is, a lower limit is about 1.5-2.0 xcexcm in spot size.
On the other hand, alignment accuracy is required for the prevention of a disadvantage that the base silicon substrate is damaged if the laser explosion regions overlap each other in blowing both fuses 202 adjacent to each other and also for the prevention of a disadvantage that in disconnecting one of fuses 202 adjacent to each other, the other is damaged or blown. Usually a lower limit of the alignment accuracy is about 0.5 xcexcm.
Thus, a lower limit of the fuse pitch of the above-described conventional fuse disconnecting method is about 2.0-2.5 xcexcm.
As a method for narrowing a pitch P of the fuses, a party of the applicants of the present application has proposed a method using a photoresist.
In the method using a photoresist, a photoresist 212 is formed on a semiconductor device shown in FIG. 11A (FIG. 12A), a laser beam 208 whose power is low enough not to cause laser explosion is irradiated to expose the photoresist 212 (FIG. 12B), the exposed photoresist 212 is developed to remove the photoresist 212 in the exposed region 214 (FIG. 12C, and a fuse 202 is removed by the usual etching process with the photoresist 212 as a mask (FIG. 12D).
According to this method, the laser beam 208 may have a power which is sufficient only to expose the photoresist 212, and it is not necessary that the power is high enough to laser explode the fuse 202 or the semiconductor substrate. Accordingly, the laser beam 208 can easily have a shorter wavelength and can have a spot size 210 which is decreased in accordance with a wavelength of the laser beam 208. Accordingly, a fuse pitch P, which is determined by a spot size 210 of the laser beam can be decreased.
However, the method using a photoresist must additionally include a photoresist application step and a photoresist development step, a fuse etching step and a photoresist releasing step. Conventionally, it has caused no trouble that the test process following completion of the wafer process has lower cleanliness in comparison with that in the wafer process clean room, but in a case that a process, such as etching or others, is performed after the test, it is necessary to perform the test process in a clean room of high cleanliness so that dust on wafers does not pollute the etching system, or an etching system which is exclusively used for the fuse disconnection is installed, which leads to higher fabrication costs rather than simple increase of fabrication steps.
As described above, in the conventional fuse disconnecting method, it is difficult to narrow a fuse pitch corresponding to increased integration of a semiconductor device while depressing increase of fabrication steps and fabrication costs.
An object of the present invention is to provide a structure of a semiconductor device including a fuse circuit which is easily higher integrated and does not add to fabrication costs and a method for fabricating the semiconductor device, and a laser system suitable to disconnect the fuses.
The present invention provides a semiconductor device and a method for fabricating the same for disconnecting a fuse by laser ablation, and a laser system suitable to disconnect fuses of the semiconductor device. Laser ablation is a phenomena that a laser beam of high intensity is irradiated to an object-to-be-irradiated to disconnect bonds of substances by energy of the irradiated laser beam and instantaneously sublimate the object-to-be-irradiated.
The conventional fuse disconnecting method using laser explosion due to absorption of a laser beam converts optical energy to vibrations of stretches, etc. of bonds of substances, i.e., to thermal energy for laser explosion, while laser ablation dissociates bonds of substances directly by optical energy, and is based on the phenomena which is quite different from laser explosion.
Due to such mechanism difference, in the laser ablation, a part a laser beam irradiated to vanishes with a boundary with respect to a part the laser beam has not been irradiated to remain in a beautiful facet. On the other hand, in the conventional laser explosion, the laser explosion takes place up to the vicinity of a part a laser beam is irradiated to, generating a number of particles and blurring the boundary between the laser beam irradiated part and a non-laser beam irradiated part. The cutting edge formed by the laser ablation is different from that formed by the laser explosion, so that the fuse disconnecting method can be distinguished by observing the cutting edge.
The laser ablation can thus beautifully remove a laser beam irradiated part but has a disadvantage that substantially all material is instantaneously removed without good controllability, with a result that not only a fuse but also a part of the semiconductor substrate therebelow are removed.
In view of this, the inventors of the present invention made earnest studies and found a material which is difficult to be sublimated by laser ablation. The inventors of the present invention are the first to have made it clear that a blocking layer of the material which is difficult to be sublimated by laser ablation is provided below the fuses to thereby stop the laser ablation on the blocking layer with good controllability.
Even in disconnecting fuses by the laser ablation, if the laser ablation can be controlled by the blocking layer, there is no risk that even with laser beams of short wavelengths, semiconductor substrates will not be damaged, as they are damaged by the conventional laser explosion. Accordingly a laser beam can have a small spot size corresponding to a wavelength of the laser beam.
In disconnecting two fuses adjacent to each other, even when both laser spots overlap each other, the blocking layer which is sufficiently thick can keep semiconductor substrates from damage. That is, a fuse pitch can be made smaller in accordance with decrease of a wavelength of the laser beam.
The laser ablation requires only a laser system to disconnect fuses and requires no additional etching system, etc., and increases neither fabrication steps and fabrication costs.
As the blocking layer for controlling the laser ablation, W (tungsten) film, for example, can be used.
That is, the above-described object is achieved by a semiconductor device comprising: a blocking layer formed on a substrate; an insulation film formed on the blocking layer; and a fuse formed on the insulation film, whereby a fuses can be disconnected by laser ablation with good controllability without damaging the base substrate. The fuses to be disconnected can be arranged at a very small pitch, which can improve integration of the fuse circuit.
The above-described object is also achieved by a semiconductor device including a memory cell region where a plurality of memory cells are formed, and a fuse circuit region where a fuse circuit for replacing a defective memory cell by a redundant memory cell is formed, the semiconductor device comprising: a blocking layer formed in the fuse circuit region; an insulation film formed on the blocking layer; and a fuse formed on the insulation film and formed of the same conducting layer as a conducting layer forming the memory cells or an interconnection layer formed in the memory cell region. The semiconductor device having this structure allows the fuses which can be disconnected by the laser ablation with good controllability to a replacement circuit to a redundant circuit of a memory device. The fuses to be disconnected by the laser ablation can be arranged at a very small pitch, which can improve integration of the memory device.
In the above-described semiconductor device, it is preferable that the fuse is formed of the same conducting layer as a metal interconnection layer formed in the memory cell region. The fuses can be formed of the same conducting layer as any of the metal interconnection layers forming the semiconductor device.
In the above-described semiconductor device, it is preferable that each of the memory cells includes a transfer transistor and a capacitor; and the fuse is formed of the same conducting layer as a gate electrode of the transfer transistor, a storage electrode of the capacitor, an opposed electrode of the capacitor or a bit line. The fuses may be formed of not only the metal interconnection layer but also of the same conducting layer as the above-described conducting layer forming the memory cells.
In the above-described semiconductor device, it is preferable that the device further comprises a cover film formed on the fuse. In the above-described semiconductor device the fuses can be disconnected by laser ablation, so that even in a case that the cover film is formed on the fuses, the fuses can be disconnected from above the cover film.
In the above-described semiconductor device, it is preferable that the device further comprises a polyimide film formed on the cover film for relaxing a stress in assembly process. In the above-described semiconductor device the fuses can be disconnected by laser ablation, so that even in a case that the polyimide film is formed on the cover film, the fuses can be disconnected from above the polyimide film.
In the above-described semiconductor device, it is preferable that the blocking layer is formed of a film including a tungsten film. Because tungsten film is difficult to be sublimated by the laser ablation, the blocking layer if formed of a film including a tungsten film, whereby laser ablation can be stopped by the blocking layer.
In the above-described semiconductor device, it is preferable that the fuse is formed of a film including a polycrystalline silicon film, an aluminum film or an aluminum alloy. These conducting materials are very easily sublimated by laser ablation, and can be used as fuses to be disconnected by laser ablation.
In the above-described semiconductor device, it is preferable that the device includes the fuse disconnected by laser ablation.
The above-described object is also achieved by a semiconductor device including a memory cell region where a plurality of memory cells are formed, and a fuse circuit region where a fuse circuit for replacing a defective memory cell by a redundant memory cell is formed, the semiconductor device comprising: a base semiconductor substrate; a layer or layers formed on the base semiconductor substrate; and a fuse formed on the layer or the layers in the fuse circuit region and formed of the same conducting layer as a conducting layer forming the memory cells or an interconnection layer formed in the memory cell region and disconnected by laser ablation, wherein a thickness of the layer or the layers is much thicker than a thickness of the fuse.
The above-described object is also achieved by a method for fabricating a semiconductor device comprising the steps of: forming a blocking layer on a substrate; forming an insulation film on the blocking layer; and forming a fuse on the insulation film. The above-described method for fabricating the semiconductor device can fabricate a semiconductor device which can disconnect the fuses by laser ablation. The fuses to be disconnected can be arranged at a very small pitch, which can improve integration of the fuse circuit.
In the above-described method for fabricating the semiconductor device, it is preferable that the method further comprises after the fuse forming step, a step of disconnecting the fuse by laser ablation. Fuses are disconnected by the laser ablation, whereby the fabrication process is not complicated, and no additional fabrication system is required. As a result, without increasing fabrication costs, the fuses can be arranged at a smaller pitch.
In the above-described method for fabricating the semiconductor device, it is preferable that in the step of disconnecting the fuse, the laser ablation is stopped by the blocking layer. The blocking layer is formed of a material which is difficult to be sublimated by laser ablation below the fuses, whereby the laser ablation can be stopped by the blocking layer with good controllability.
In the above-described method for fabricating the semiconductor device, it is preferable that in the step of disconnecting the fuse, the fuse is disconnected by a laser beam having a wavelength of not more than 500 nm. It is not necessary that the laser ablation considers absorption of a laser beam by the base substrate, so that laser beams of a below 1 xcexcm-wavelength range, which is the absorption range of the substrate, can be used. By using such laser beams of short wavelengths the laser beams can have reduced spot sizes, whereby the fuses can be arranged at a small fuse pitch.
In the above-described method for fabricating the semiconductor device, it is preferable that the laser beam is third or more harmonics of a Nd:YAG laser or third or more harmonics of a Nd:YLF laser.
In the above-described method for fabricating the semiconductor device, it is preferable that the method further comprises after the fuse forming step, a step of forming a cover film for covering the fuse. In the laser ablation the layers are sublimated sequentially from above, so that even in a case that the cover film is formed on the fuses, the fuses can be disconnected from above the cover film.
In the above-described method for fabricating the semiconductor device, it is preferable that the method further comprises a step of forming a polyimide film for relaxing a stress in assembly process. Even in a case that the polyimide film is formed on the cover film, the fuses can be disconnected also from above the polyimide film.
In the above-described method for fabricating the semiconductor device, it is preferable that in the step of forming the blocking layer, the blocking layer including a tungsten film is formed.
The above-described object is also achieved by a laser system for disconnecting by laser ablation a fuse of the semiconductor device including a blocking layer formed on a substrate, an insulation film formed on the blocking layer and the fuse formed on the insulation film, the laser system comprising: a laser resonator for oscillating a laser beam having an oscillation wavelength of not more than 500 nm and an energy density sufficient to disconnect the fuse by laser ablation; a lens mechanism for condensing the laser beam emitted by the laser resonator into a required spot size; and an alignment mechanism for irradiating the laser beam outputted by the laser resonator to a required position on the semiconductor device. The laser system having this structure applies a laser beam at an arbitrary position on a wafer to disconnect the fuses by laser ablation.
In the above-described laser system it is preferable that the laser resonator outputs third or more harmonics of a Nd:YAG laser or third or more harmonics of a Nd:YLF laser.